Conditions affecting the proper functioning of the mitral valve include, for example, mitral valve regurgitation, mitral valve prolapse and mitral valve stenosis. Mitral valve regurgitation is a disorder of the heart in which the leaflets of the mitral valve fail to coapt into apposition at peak contraction pressures, resulting in abnormal leaking of blood from the left ventricle into the left atrium. There are a number of structural factors that may affect the proper closure of the mitral valve leaflets. For example, many patients suffering from heart disease experience dilation of the heart muscle, resulting in an enlarged mitral annulus. Enlargement of the mitral annulus makes it difficult for the leaflets to coapt during systole. A stretch or tear in the chordae tendineae, the tendons connecting the papillary muscles to the inferior side of the mitral valve leaflets, may also affect proper closure of the mitral annulus. A ruptured chordae tendineae, for example, may cause a valve leaflet to prolapse into the left atrium due to inadequate tension on the leaflet. Abnormal backflow can also occur when the functioning of the papillary muscles is compromised, for example, due to ischemia. As the left ventricle contracts during systole, the affected papillary muscles do not contract sufficiently to effect proper closure.
Mitral valve prolapse, or when the mitral leaflets bulge abnormally up in to the left atrium, causes irregular behavior of the mitral valve and may also lead to mitral valve regurgitation. Normal functioning of the mitral valve may also be affected by mitral valve stenosis, or a narrowing of the mitral valve orifice, which causes impedance of filling of the left ventricle in diastole.
Typically, treatment for mitral valve regurgitation has involved the application of diuretics and/or vasodilators to reduce the amount of blood flowing back into the left atrium. Other procedures have involved surgical approaches (open and intravascular) for either the repair or replacement of the valve. For example, typical repair approaches have involved where the leaflets of the valve are either made to cinch or portions of the dilated annulus are resected.
Cinching of the annulus has been accomplished by the implantation of annular or peri-annular rings which are generally secured to the annulus or surrounding tissue. Other repair procedures have also involved cinching or clipping of the valve leaflets into partial apposition with one another as well. Alternatively, more invasive procedures have involved the replacement of the entire valve itself where mechanical valves or biological tissue are implanted into the heart in place of the mitral valve. These are conventionally done through large open thoracotomies and are thus very painful and require long recovery periods.
However, with many repair and replacement procedures the durability of the devices or improper sizing of annuloplasty rings or replacement valves may result in additional problems for the patient. Moreover, many of the repair procedures are highly dependent upon the skill of the cardiac surgeon where poorly or inaccurately placed sutures may affect the success of procedures.
Mitral valve replacement, compared with aortic valve replacement, poses unique anatomical obstacles, rendering percutaneous mitral valve replacement significantly more involved and challenging than aortic. First, unlike the relatively symmetric and uniform aortic valve, the mitral valve annulus has a non-circular oval or kidney-like shape, and may be of unpredictable geometry, often times lacking symmetry. Such unpredictability makes it difficult to design a mitral valve prosthesis having the ability to conform to the mitral annulus. Lack of a snug fit between the leaflets and/or annulus and the prosthesis leaves gaps therein, creating backflow of blood through these gaps. Placement of a cylindrical valve prostheses, for example, may leave gaps in commissural regions of the native valve, potentially resulting in perivalvular leaks in those regions,
In addition to its irregular, unpredictable shape, the mitral valve annulus lacks a significant amount of radial support from surrounding tissue. The aortic valve, for example, is completely surrounded by muscular tissue, helping to anchor a prosthetic valve by providing native structural support. The mitral valve, on the other hand, is bounded by muscular tissue on the outer wall only. The inner wall of the mitral valve is bounded by only a thin wall of tissue separating the mitral valve annulus from the inferior portion of the aortic tract. As a result, significant radial forces on the mitral annulus, such as that imparted by expanding stent prostheses, could lead to collapse of the inferior portion of the aortic tract with potentially fatal consequences.
The chordae tendineae of the left ventricle may also present an obstacle in deploying a mitral valve prosthesis. This is unique to the mitral valve since aortic valve anatomy does not include chordae. The maze of chordae in the left ventricle makes navigating and positioning a deployment catheter that much more difficult in mitral valve replacement and repair. Deployment and positioning of a prosthetic valve or anchoring device on the ventricular side of the native valve is also complicated by the presence of the chordae.
Given the difficulties associated with current procedures, there remains the need for simple, effective, and less invasive devices and methods for treating dysfunctional heart valves.